374 CHEMICAL ENGINEERING Figures 19 and 20 shows also a similar type of evaporator as constructed by Mantius and Badger, but in both cases the liquor is recirculated from the vapor body to the liquor chest, and is only discharged when it has reached the correct density. FIG. 20.—Badger rapid-circulation evaporator. AUXILIARIES Salt Filters.—A great many of the solutions to be handled in the chemical and allied industries separate salts during the concentration, either as a main product or by-product. In any case, it is necessary to separate this salt from the liquor, and this is accomplished by either discharging the mixture of liquor and salt into an open salt filter from where the liquor is returned to the evaporator, or sent to storage; or by separating the two products in a closed salt filter; or by discharging the mixture of the two into a trough and a rotary salt filter, where the salt will stick to the surface of the drum, and the solution is discharged by the pump. It is beyond the scope of this article to go into further details, and it is only to be mentioned that considerable progress has been made during the last few years in the handling of these salts, which ordinarily required considerable labor and expense. Condensing System.—In the majority of cases, the vapors coming from the last effect must be condensed in some type of condenser, depending on the vacuum under which the plant is to be operated. The condenser may be of any standard design, but it should be borne in mind there are sometimes large fluctuations in the amount of vapor going to the condenser, and also that an evaporat- NOTE.—A list of the most prominent manufacturers of these evaporators will be found in "The Chemical Catalog. "